1. Field of the Disclosure
This disclosure relates to an organic light emitting display (OLED) device.
2. Description of the Related Art
OLED devices use an organic light emission layer that emits light through the recombination of electrons with electrical holes. Such OLED devices corresponding to a self-luminous display device are considered to be next generation display devices due to their high brightness, low drive voltage and possible slimness.
An OLED device includes a plurality of pixel elements. Each of the pixel elements includes a pixel configured with an organic light emission layer between an anode and a cathode, and a pixel circuit configured to drive the pixel. The pixel circuit is configured to include a switching transistor, a capacitor and a driving transistor. The switching transistor receives a scan pulse and charges a data voltage into the capacitor. The driving transistor controls an amount of electrical current to be applied to the pixel based on the data voltage charged in the capacitor, thereby adjusting a gray level of the pixel.
A data driver included in a driver circuit of the OLED device subdivides a plurality of reference voltages from an external gamma voltage generator into gray scale gamma voltages. Also, the data driver converts digital data into an analog data signal (more specifically, a voltage signal or a current signal) using the gray scale gamma voltages. The OLED device adjusts the brightness of the OLED device by adjusting the most significant reference voltage based on a brightness control command from a user.
FIG. 1 is a data sheet illustrating the characteristics of gamma voltages conventionally used for driving OLED devices.
Referring to FIG. 1, the conventional gamma voltage generator (e.g., within the data driver) is configured with a plurality of input gamma tabs (for example, zeroth through ninth gamma tabs) with serially connected resistors between each tab. The ninth gamma tab receives the highest reference voltage on the basis of a power supply voltage VDD. The zeroth gamma tab receives the lowest reference voltage on the basis of a ground voltage VSS. The reference voltages received by the input gamma tabs decrease in order from the ninth gamma tab to the zeroth gamma tab. The gamma voltage generator also has output gamma tabs. The output gamma tabs output gamma voltages that decrease in voltage from the highest order (e.g. 255th) to the lowest order (e.g. 0th) tab. The output gamma voltages also correspond to gray scale levels 255 through 0.
In the first related art “-●-”, the reference voltages are sequentially lowered as the orders of the gamma tabs are lowered (the ninth gamma tab is the highest order tab, the zeroth gamma tab is the lowest order tab). The lowest gamma voltage is used for deriving a lowest gray scale data signal with a lowest voltage, in order to realize black brightness. Also, the highest gamma voltage is used for deriving a highest gray scale data signal with a highest voltage, in order to realize white brightness. In other words, the gamma voltage is used to drive the pixel to black brightness.
The first related art “-●-” has a gamma characteristic as a normal gamma curve of 2.2 shown in FIG. 1. To this end, the first related art raises the reference gamma voltages by a fixed level according to a sequence progressing from the zeroth gamma tab to the ninth gamma tab. The first related art also raises the voltages of the gray scale data signals in the same manner as the reference gamma voltages.
As such, in the first related art, the lowest gamma voltage is used for realizing black brightness, and the highest gamma voltage is used for realizing white brightness. In other words, the lowest gamma voltage corresponds to a gray scale level of “0” (black brightness), and the highest gamma voltage corresponds to a gray scale level of “255” (white brightness).
Particularly, the first related art physically separates zeroth and first gamma output tabs, which output the gamma voltages opposite to the gray scales of “0”and “1”. Separating the zeroth and first gamma output tabs from each allows the gamma voltage output by the zeroth tab to have a voltage level that corresponds to substantial black brightness.
The second related art “-▪-” also provides the same gamma voltages as the first related art. However, the second related art enables not only the lowest gamma voltage to be used for deriving a lowest gray scale data signal with the highest voltage, but also the highest gamma voltage to be used for deriving a highest gray scale data signal with the lowest voltage, unlike the first related art.
In other words, the second related art “-▪-” allows the voltages of the gray scale data signal to be in inverse proportion to the gamma voltages being output from gamma output tabs. This is due to the first related art being configured to drive a NMOS pixel, and the second related art being configured to drive a PMOS pixel.
As such, in the second related art, as the order of the gamma output tab becomes higher, the value of the gray scale is lowered from “255”to “0”. More specifically, the lowest gamma voltage generated at the most significant gamma output tab (e.g. the 255th output tab) corresponds to the lowest gray scale data signal which has the highest voltage and is used for realizing black brightness. Also, the highest gamma voltage generated at the least significant gamma output tab (e.g. the zeroth output tab) corresponds to the highest gray scale data signal which has the lowest voltage and is used for realizing white brightness.
However, the second related art reversely matching the gamma voltages to the gray scale data signals causes the deterioration of brightness in a low gray scale domain, unlike the first related art.
FIG. 2 is a data sheet illustrating brightness characteristics of OLED devices according to the related arts. FIG. 3 is a data sheet illustrating the characteristics of data voltages of OLED devices according to the second related art.
Referring to FIG. 2, when the OLED device of the first related art is driven, black brightness rises steeply between the gray scales of “0” and “1” and then rises slowly from the gray scale of “1” to the gray scale of “31”. This results from the fact that the zeroth and first gamma input tabs (and also the zeroth and first gamma output tabs) are physically separated from each other in order to realize black brightness.
On the other hand, referring to FIGS. 2 and 3, black brightness provided by the OLED device of the second related art, which uses the gamma voltages that are inverted from those in the first related art, is linearly varied from the gray scale of “0” to the gray scale of “31” without the steep increase between the gray scales of “0” and “1”. This is because the ninth and eighth input gamma tabs are connected to each other through resistors.
Due to this, the OLED device of the second related art provides lower brightness in a gray scale range of 1-31, compared to that of the first related art, as shown in FIG. 2.
Also, although it is not shown in the drawings, the second related art includes resistors connected between the ninth and eighth gamma input tabs. In other words, the ninth and eighth gamma input tabs in the second related art are not separated from each other. As such, the high data voltages corresponding to the gray scales of 0 through 31 increase the quantity of current.
Because the eighth and ninth tabs are not separated, the current increment in the ninth and eighth gamma tabs causes high power consumption. Due to this, a large quantity of heat is generated in the gamma voltage generator, and reduces the life span of the components in the gamma voltage generator.